A known seat reclining apparatus for a vehicle intended for inclining a seatback relative to a seat cushion is disclosed in JP07-255553A. The seat reclining apparatus disclosed includes a lower plate attached to a seat cushion and having internal gear teeth, and an upper plate attached to a seatback and having external gear teeth. The external gear teeth engage with the internal gear teeth in a state where the total number of the external gear teeth is smaller than that of the internal gear teeth. An angle of inclination of the upper plate relative to the lower plate is sequentially adjustable by a circumferential movement of a position at which the internal gear teeth and the external gear teeth engage with each other (i.e. engagement position). Then, a combination of a pair of wedge members provided between the lower plate and the upper plate, and a cam member is used as a mechanism for rotating or shifting the engagement position of the internal gear teeth and the external gear teeth. The cam member is rotated by an operating handle, and then the wedge members are rotated to press the internal gear teeth of the lower plate and the external gear teeth of the upper plate in a predetermined direction so that both gear teeth engage with each other, thereby achieving a rotation or shifting of the engagement position.
However, according to JP07-255553A, in the cases where the seat reclining apparatus is applied to a belt integrated seat in which a seatbelt and its peripheral structure are integrated into a seat, an absolute strength of the seat is weak against a large load added to the seatback in the event of a vehicle collision and thus the seat may not endure the load at that time.
Meanwhile, a seat reclining apparatus for a vehicle intended for higher strength is disclosed in JP3018566B2. The seat reclining apparatus disclosed includes two reclining mechanisms attached to each other in a back-to-back manner so as to be arranged in a mirror image relationship. Gear shapes (i.e. internal gear teeth and external gear teeth) of the two reclining mechanisms are differently formed so that a backlash between the internal gear teeth and the external gear teeth of one of the reclining mechanisms is specified greater than that between the internal gear teeth and the external gear teeth of the other one of the reclining mechanisms. Accordingly, in the case of normal operation, the internal gear teeth and the external gear teeth of one of the reclining mechanisms engage with each other while the internal gear teeth and the external gear teeth of the other one of the reclining mechanisms are not in contact with each other, thereby preventing unnecessary increase of an operation force. In addition, in the event of the vehicle collision, the internal gear teeth and the external gear teeth of the other one of the reclining mechanisms make contact with each other upon deformation of the internal gear teeth and the external gear teeth of one of the reclining mechanisms, thereby causing both the reclining mechanisms to receive the load at that time.
According to the seat reclining apparatus disclosed in JP3018566B2, the shapes of the respective internal gear teeth and the external gear teeth of two reclining mechanisms are different and thus two types of parts are required for engagement of the gear teeth, which leads to an increase of the number of parts. In addition, even for the internal gear teeth and the external gear teeth engaging with each other or making contact with each other in the normal operation, a small backlash is required therebetween for the purposes of a smooth engagement operation or prevention of effect on the operation due to gear accuracy. Accordingly, looseness is generated in the seatback, which may cause a decrease of quality feeling.
Thus, a need exists for a seat reclining apparatus for a vehicle that can improve strength thereof without deterioration of quality feeling.